Northern Rockies Skies for October: Pegasus, the ‘Winged Horse’

September 26, 2013

A monthly look at the night skies of the northern Rocky
Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay
Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

In the autumn night skies, Pegasus, the winged horse, is a prominent
constellation overlooking the October harvest. Named after the Greek
mythological figure, Pegasus sprang from the neck of the Medusa, when she was
beheaded by Perseus. Pegasus is noted by a great square, outlined by its three
brightest stars -- Markab, Scheat and Algenib -- and Alpheratz, the brightest
star in Andromeda.

Although Pegasus seems to be devoid of any interesting naked-eye objects, it
does have a few very interesting deep sky objects -- most notably, the classic globular
cluster M 15; Stephan’s Quintet, a region of five closely grouped galaxies that
are in the process of evolving; and Einstein’s Cross, which consists of four
separate images of the same quasar that have been formed by the gravitational
lensing of a nearby galaxy.

Planet and meteor alert: Saturn,
Mercury and Venus can be seen right after sunset on the western horizon.
Jupiter rises around midnight and Mars around 4 a.m. The Draconid meteor shower
peaks around Oct. 7-8 and the Orionids around Oct. 21-22. Both are best seen
after midnight.

Last month, we discussed the atmosphere of Venus -- Earth's
"sister planet" -- and it turned out not to be sister-like at all.
Venus has very high pressures and temperatures compared to Earth, and its
surface attributes are similarly foreign.

Of the many orbiter and lander spacecraft sent to Venus, the
most productive was the Magellan spacecraft (1989-1994), which mapped the planet’s
entire surface from orbit, using radar to penetrate the dense Venusian cloud
layer. The URL above includes a movie of the radar-mapped surface with Venus
rotating. The radar map resolves features down to a scale of about one
kilometer.

The map shows that Venus is about 80 percent covered by
smooth volcanic plains, with the remainder being two raised areas, or
"continents,” one each in the planet’s northern and southern hemispheres. The
continents are named Ishtar Terra (about the size of Australia) after the
Babylonian goddess of love; and Aphrodite Terra (the size of South America)
after the Greek goddess of love.

Venus has several smaller types of features, unlike any
found on other planets in our solar system. These include dome or
pancake-shaped areas ranging 20-50 kilometers in diameter, suggestive of
ancient welling up of lava; ray-like fractures called "novae"; more
complex radial and concentric networks resembling spider webs; and some lava
channels more than 6,000 kilometers long.

Venus has few impact craters, most likely because its surface
is relatively young. Its surface is less than 800 million years old -- more
recent than the late heavy bombardment phase (roughly 4 billion years ago)
hypothesized to have last cratered the Earth's moon and the other terrestrial
planets of the solar system.

The Venusian surface is still much older than the Earth's
average surface age of 100 million years, because Venus does not have plate
tectonic dynamics that would continually refigure the surface. While mostly
quiescent, Venus has 167 volcanoes larger than 100 kilometers in diameter, some
of which may have been active in the recent past, as evidenced by sulfur in the
atmosphere.